There have heretofore been known the processes for producing high-performance styrenic polymers having a high degree of syndiotactic configuration in high yield by allowing a reaction product of an aluminoxane with a transition metal complex to act on styrene. (Refer to Japanese Patent Application Laid-Open Nos. 187708/1987, 179906/1988, 241009/1988, etc.) The catalyst system containing an aluminoxane has the advantage of high activity, but it is required in the case of methylaluminoxane which has a particularly high activity to use a large amount thereof against a transition metal compound. In particular in order to obtain a styrenic polymer having a molecular weight of 600,000 or less, it is necessary to add a large amount of a Lewis acid to the reaction system or set the temperature at the time of polymerization to a high level. The above-mentioned necessity is responsible for such problems as a large amount of ash remaining in the product polymer and deteriorated catalytic activity.
As a catalyst free from an aluminoxane for producing a styrenic polymer and a process for producing the same without the use of an aluminoxane, there is disclosed in Japanese Patent Application Laid-Open No. 249504/1992, a method for polymerizing a styrenic monomer by the use of a catalyst system which comprises a transition metal compound and a non-coordination ionic compound. However in the case of producing a polymer, a catalyst system not containing an aluminoxane causes difficulty in controlling the molecular weight of the product polymer as compared with a catalyst system containing an aluminoxane. In particular, in order to obtain a styrenic polymer having a weight-average molecular weight of 600,000 or less, it is still necessary to add a large amount of a molecular-weight depressant to the polymerization system or to set the polymerization reaction temperature at a high level. Such necessity brings about a decrease in the catalytic efficiency and an increase in the amount of residual ash, thereby increasing the polymer production cost and deteriorating the physical properties of the resulting polymer.
Under such circumstances, it is an object of the present invention to provide a catalyst for producing a vinyl-based polymer capable of efficiently producing a vinyl product-addition polymer such as polystyrene and polyolefin as well as a process for efficiently producing in a low production cost, an aromatic polymer having a low molecular weight and a high degree of syndiotactic configuration.